1. Field
Semiconductor fabrication.
2. Description of Related Art
Photolithography is a technique for transferring an image rendered on one media onto another media photographically. Photolithography techniques are widely used in semiconductor fabrication. Typically, a circuit pattern is rendered as a positive or negative mask image which is then projected onto a silicon substrate coated with photosensitive materials. Radiation impinges on the masked surface to chemically change those areas of the coating exposed to the radiation, usually by polymerizing the exposed coating. The unpolymerized areas are removed, being more soluble in the developer than the polymerized regions, and the desired image pattern remains.
As circuits continue to miniaturize in scale, chip manufacturers have started using radiation with wavelengths shorter than the visible light. Typically, optical lithography systems use radiation at wavelengths such as 365 nanometers (nm), 248 nm, 193 nm, and 157 nm. Advanced non-optical lithography systems with shorter wavelengths such as extreme ultraviolet (EUV) are being actively researched for printing complex patterns in the extreme submicron range. However, the problem of diffraction-limited optics remains with these technologies.
Another recent technique developed by chip manufacturers to increase the resolution and contrast of optical lithography involves the use of immersion technology. Immersion lithography is a resolution enhancement technique that interposes a liquid medium between optical components and the wafer surface, replacing the usual air gap. This liquid medium has a refractive index greater than one. With the 193 nm wavelength, a typical liquid medium is ultra-pure, degassed water. Although the immersion technology may enable the reduction of circuit feature size, upgrade to an immersion lithography system is very costly. Moreover, many defect issues induced by the immersion technology are still lacking a feasible solution.